The general trend in the electronics packaging technology is toward miniaturization both of discrete components and integrated circuits (ICs). A good example is in the notebook and sub-notebook computer field. As more functions are integrated on an IC, more connections off the chip are required, and more circuit traces are needed to interconnect them.
With the advancement to very large scale integration (VLSI) in IC circuit design the requirement for dense circuitry has risen almost exponentially. Additionally, Surface Mount Technology (SMT) has provided even higher printed circuit board (PCB) densities by reducing the required sizes of components, input/output (I/O) connections, and the width of line traces. SMT allows the use of both sides of a PCB since the electrical components are soldered to the surface only. This has stimulated further developments in laminating successive layers of electrical circuit traces and layers of insulators.
As a result of these technological advancements in microelectronics a great deal of effort has been placed on increasing "connectivity," or the ability of a printed wiring process to provide electrical connection to all nodes in a circuit. The result is a concurrent reduction in conductor line (trace or line ratio) width, and an increase in the number of conductor layers. Therefore, a dramatic increase in capability for circuit density and complexity has resulted. The increase in conductive circuit layers has led to development of Multilayered Circuit Boards (MLBs). MLBs provide a reliable cost effective engineering solution for microcircuit packaging.
FIG. 2 is an illustration of a four-layered MLB. By definition, MLBs have three or more circuit layers. To make a board of this sort, networks of passive circuit elements are deposited in predetermined geometric patterns on surfaces of insulating substrates. The substrates are later joined to provide a single structure with multiple layers of circuitry.
A widely practiced method or making MLBs is by bonding, or laminating, the separate layers of patterned, pre etched, plated-through-hole, copper-clad laminates together. A plated through hole (PTH) 67 is the electrical pathway for interconnection of "buried" layered circuits in the MLB. A typical multilayer circuit board is made up of successive layers of conductive circuits such as layers 63A, 63B, 65A & 65B that have typically been phototactrally traced and chemically etched, epoxy-resin-glass dielectrics 61, and epoxy-glass that has been dried and partially cured. This partially cured epoxy-glass serves as the bonding material (glue or prepreg) to hold the board together.
The dual copper plate with an epoxy-glass, Invar layer 60 along with the dielectric 61 is railed the substrate. The copper layers 63A & 63B in this example provide power circuitry to the exposed trace circuits. PCB substrate material is an important part of overall structural integrity for any MLB and must closely match the conductive trace layer's thermal expansion rate. The dielectric constant of the substrate is one factor determining the board's required conductive trace width. The trace width is the physical dimension measured across one conductive circuit path in the plane of the printed circuit board. Each individual circuit trace is the electrical pathway to one of many circuits to one or more electrical components.
A commonly used industrial substrate is FB4 board. The stack of layers is typically heated under pressure until the glue or preppeg is cured. MLBs inner layers of circuits 63A and 63B in FIG. 2, formed by printing and etching, can also be interconnected by drilled and plated through holes, and become buried interconnections or buried vias.
The concept of "real estate" in designing a PCB is an important design parameter. The MLBs also provide the designer an opportunity to use both sides of the PCB to accommodate the increased number of components and provide required air flow fop cooling. Perpendicular plane mounting in relation to a mother board, a shown in FIG. 1A, is a common practice for uniform convection cooling and allows higher circuit densities than would otherwise be useful. These subassemblies represent one packaging hierarchy in general design criteria.
An integrated circuit such as a DRAM is typically packaged as a module 19, as shown in FIGS. 1A and 1B, and then assembled on a multilayer PCB with other modules and possibly with other components, such as resistors, capacitors, as shown in FIG. 1A. The PCB, also known as a "card", has a standardized I/O interface that fits into a plug receptacle 41 on the motherboard, as shown in FIG. 1B. This also provides for a larger "virtual" board that minimizes connectors and keeps the motherboard backplane smaller. The multilayer card is a second level in packaging hierarchy that allows for higher I/O connection count per board and shorter circuit paths between many components. Thus the system has higher speeds to accommodate high frequency CPUs. The overall system conserves power by reducing the number of active component in the system. Such a DRAM card is called a memory module in the art.
DRAM is high-speed memory that holds binary instructions and data which is transferred between hardware units by machine control routines. DRAM modules are present in most sophisticated computerized machinery. In particular, the computer field has been memory driven in system and software design. For example, today's portable computer technologies compete with the performance of desk-top microprocessor units.
To capture a market segment for portable computing, the industry must provide smaller, lighter-weight notebook and sub-notebook computers. These computers also need to incorporate high frequency CPUs used in powerful desk-top units and the memory to support state-of-the-art software applications. To provide the needed memory, DRAM occupies more and more valuable real estate in portable computers.
FIGS. 3A and 3B are sectional views of idealized four-layer memory module MLBs in current art. FIG. 3A is a one-sided memory module 40 comprising packaged DRAM ICs such as DRAM 42 soldered to surface mounting pads, and FIG. 3B shows a double-sided memory module 44. Two sided memory module 44 takes up about 50% more volume then single-sided module 40.
What is needed to provide a significantly higher circuit density to further miniaturize notebook and sub-notebook computers is a more space efficient PCB system for memory modules and discrete electronic components. The module miniaturization of DRAM could then be transferred to smaller motherboard designs and thus, faster circuitry. Such a system should address the density of all related electronic components and the heat dissipation required. In such a system, a space-saving module would preferably be amenable to existing manufacturing techniques and materials.